The present invention relates to epicyclic gearboxes, and more particularly, to a planet gear carrier in an epicyclic gearbox.
Epicyclic or planetary gearboxes are frequently used in gas turbine engines for their compact designs and efficient high gear reduction capabilities. Planetary gear trains are well known, and are generally comprised of three gear train elements: a central sun gear, an outer ring gear with internal gear teeth, and a plurality of planet gears supported by a planet carrier between and in meshing engagement with both the sun gear and the ring gear. All three gear train elements share a common longitudinal central axis, about which at least two of them rotate. An advantage of planetary gear trains is their versatility. A rotary input can be connected to any one of the three elements. Holding one of the remaining two elements stationary with respect to the other two, permits the third to serve as an output.
In gas turbine engine applications, where a speed reduction transmission is required, the central sun gear generally provides rotary input from the powerplant, and the outer ring gear is held stationary. The planet gear carrier therefore provides torque output at a reduced rotational speed.
However, certain shortcomings do exist with planetary drive trains. For example, as with many mechanical elements that transfer torque, a small but nevertheless significant amount of torsional deflection commonly occurs due to the elasticity of the material of the carrier, as a result of twist between upstream and downstream plates of the planetary gear carrier, when the gear train is under load. The plates of the planet gear carrier twist relative to one another around the central axis, causing the axles of the planet gears to lose parallelism with the central axis of the planetary carrier. This torsional deflection results in misalignment at gear train journal bearings and at the gear teeth mesh point, which leads to efficiency losses and reduced life of the parts. Additionally, increased oil flow is required to the journal bearings to compensate for the misalignments caused by torsional deflections of the planet carrier plates.
Attempts to address this problem of planetary carrier torsional deflection are known. U. S. Pat. No. 5,466,198 issued Nov. 14, 1995 to McKibbin et al., for example, clearly sets out the problem and proposes a planetary gear train drive system which isolates the planetary carrier from torsional deflections. A torque frame or torque transfer structure is connected to a rotating load, such as a bladed propulsor. Pivotal joints, circumferentially disposed with respect to the carrier, each pivotable about a radial axis, connect axially extending arms of a torque frame to the planetary carrier. The pivotal joints, which are vital to the invention of McKibbin et al., permit the planetary carrier to be isolated from torsional deflections. While McKibbin et al. do provide a device that eliminates planetary carrier torsional deflections, the planetary carrier system disclosed is of significant complexity. Both a low number of parts and low weight are characteristics vital in aircraft applications. Also, added parts, especially involving pivotable joints, increases the possibility of reliability problems.
Therefore, there remains a need for a simple, compact, device capable of transferring torque while eliminating twist within a planetary carrier.
It is an object of the present invention to provide an improved planetary gear train.
It is an object of the present invention to provide a torque transfer device for use in a planetary gear train.
It is another object of the present invention to provide a planetary carrier capable of torque transfer with minimal twist occurring between the upstream and downstream plates of a planetary carrier.
Therefore, in accordance with the present invention, there is provided a torque transfer assembly adapted for use in a planetary gear train, the gear train including a sun gear rotatable about an axially extending central axis, a concentric stationary outer ring gear, and a plurality of planet gears mechanically intermediate said sun gear and said ring gear and in meshing engagement therewith, the plurality of planet gears adapted for receiving torque input from the sun gear, said torque transfer assembly comprising: a planetary carrier, rotatable about said axially extending central axis and adapted to rotatably support said plurality of planet gears on a plurality of axles between first and second axle ends, said first and second axle ends defining first and second planes respectively, said plurality of axles being parallel to the central axis and the first and second planes being perpendicular to the central axis, the planet gears being circumferentially located on the planetary carrier about the central axis, the carrier having a first connecting member extending therefrom; and a torque transfer coupling adapter, disposed concentrically with said planetary carrier and rotatable therewith, said torque transfer coupling adapter having a central torque output element and a second connecting member extending therefrom, said second connecting member adapted to be engaged with said first connecting member to structurally join the coupling adapter and the planetary carrier, said first and second connecting members being structurally joined together between said first and second planes.